1. Field of the Invention
This invention relates to a heat stable fiber forming polyester and to a new and novel process for preparing it. More particularly, this invention relates to an improved linear high molecular weight heat stable polyester especially suitable for preparing fibers which have excellent resistance to degradation when utilized in commercial articles, such as tires, industrial belting, etc., wherein a high degree of heat is built up during use.
2. Description of the Prior Art
High molecular weight polyethylene terephthalate fiber forming polyesters are well known. They are prepared commercially either by the ester interchange reaction between dimethyl terephthalate and ethylene glycol or by the direct esterification process wherein terephthalic acid is reacted directly with ethylene glycol. These products and processes are well documented in U.S. patents such as U.S. Pat. Nos. 2,465,319; 3,050,533; 3,051,212; 3,427,287 and 3,484,410 which cover not only the basic products and processes but many improvements thereon.
Polyethylene terephthalate fibers and cords are known to exhibit excellent dimensional stability, that is, low extension or growth during service, as well as to have a high resistance to thermal degradation; however, in pneumatic tires and industrial belts under high speed conditions under heavy load, loss of tensile strength is experienced due to high temperature reactions occurring under such conditions. In efforts to remedy this problem, most research in this field has been directed to producing a high molecular weight linear polyester having a low content of free carboxyl groups. The following patents are pertinent:
U.S. Pat. No. 2,863,854 to Wilson is directed to reacting the carboxyl groups of polyesters with orgnic carbonates, such as ethylene carbonate, in the presence of a catalyst including salts of mineral acids such as zinc chloride. The reaction creates a polyester having fewer carboxyl groups.
Applicants have discovered a very effective catalyst for the above reaction.
U.S. Pat. No. 3,657,191 to Rudolph Titzmann et al. is directed to a process for the manufacture of linear polyesters having an improved stability with respect to compounds with active hydrogen. Polyesters of this type are obtained by reacting polyesters with ethylene carbonates or monofunctional glycidyl ethers.